I'm going to use a coordinate system: Frank's plane starts in (0,0) and Gordon's plane starts in (0, -300). The y-axis is pointing North and the x-axis is pointing East. The variable t represents the elapsed time and is measured in hours.
The course of Frank's plane on the x-axis is:
The course of Gordon's plane on the y-axis is:
The distance between both planes is calculated using the formula for the distance between two points:
The rate of change is d'(t):
to a) At 9:15 the elapsed time is t=0.25 hours. Plug in this value into d'(t) and you'll get the rate of change:
to b) The approach will cease if the rate of change equals zero:
when the planes are 233.38 km apart(is that the right expression?)